There are various references.  The one I like is the following, particularly Remark 1.3.3.

MR2007376 (2005b:14049) Reviewed
Abramovich, Dan(1-BOST); Corti, Alessio(4-CAMB); Vistoli, Angelo(I-BOLO)
Twisted bundles and admissible covers. (English summary)
Special issue in honor of Steven L. Kleiman.
Comm. Algebra 31 (2003), no. 8, 3547–3618.
14H10 (14A20 14H30)
http://arxiv.org/pdf/math/0106211.pdf